The invention relates to a component operating with bulk acoustic waves that comprises a thin-film resonator, which is also called a (Thin Film Bulk Acoustic Wave Resonator (FBAR) or Bulk Acoustic Wave (BAW) resonator.
Components such as these can be used in particular as filters in terminals for mobile telecommunications.
FIG. 1 schematically illustrates a cross section of a known BAW component with two thin-film resonators PR1 and PR2 that are arranged, for example, in parallel branches of a filter circuit. The thin-film resonators each contain an upper electrode E21 and E22, and share a lower electrode, E1. A piezoelectric layer PS is arranged in between. The resonators PR1, PR2 are arranged on a carrier substrate TS. An acoustic mirror, that is in each case arranged between each resonator and the carrier substrate, comprises an alternating sequence of layers of high and low acoustic impedance, and prevents the acoustic wave from emerging from the resonator in the direction of the carrier substrate TS. The thickness of each of the mirror layers is in each case approximately ¼ of the wavelength of the acoustic wave in the given material. A first acoustic mirror, which is arranged under the resonator PR1, comprises an alternating sequence of layers with low acoustic impedance (LZ1, LZ2, LZ3) and structured layers with a high acoustic impedance, which have structures HZ11 and HZ21. A second acoustic mirror, which is arranged under the resonator PR2, is formed by an alternating sequence of layers with low acoustic impedance (LZ1, LZ2, LZ3) and structures HZ12, HZ22 of the structured layers with high acoustic impedance.
The mirror layers with a high acoustic impedance may be conductive layers. In the present invention, high acoustic impedance layers are typically conductive. As a rule, layers such as these have to be structured in order to reduce disturbing capacitive couplings via a continuous conductive layer when there are two or more resonators that are arranged on a common acoustic mirror.
An arrangement of two resonators which are electrically connected to one another via their upper electrodes on a common acoustic mirror with continuous conductive layers is known, for example, from U.S. Published Application No. 2002/0084873 A1. The continuous conductive mirror layer in this case couples the lower electrodes of the resonators, while these resonators are electrically connected to one another via their upper electrodes. The coupling thus bridges the resonators in the corresponding signal path.
Components that are used in mobile radio terminals must, on the one hand, have a low insertion loss for the signal in the passband, and on the other hand must have high selectivity in predetermined stopbands.